1. Field of the Invention
The invention relates to an abrasive pad and to a process for the abrasive machining of surfaces, in particular of semiconductor wafers, having a polymer matrix of a defined water-solubility.
Processes for the abrasive machining of surfaces are in widespread use, for example in the production of electronic memory elements. Elements of this type are generally constructed in layers from different materials. A build-up or patterning step, which may consist, for example, in an etching, sputtering or oxide deposition step, very often has to be followed by a planarization step, since the layer structure does not generally satisfy the highly accurate surface demands which are required or reproduces the topography of a wiring plane lying at a lower level even though the intention is actually to produce a planar surface. Chemical mechanical polishing (CMP) has gained widespread acceptance for planarization.
In CMP, surface regions lying at higher levels are accurately removed, in a manner which is as topography-selective as possible, by the interaction of liquid chemicals and abrasive bodies moving on the surface, such as for example polishing grains which can move freely or are fixed in a polishing cloth. Often, further material has to be removed after the planarization, the intention being, for example, for the removal of material to take place uniformly over the entire surface. In some applications, material-specific removal is also desired. In that case, a distinction is drawn between higher regions of a lower layer which have been uncovered by the CMP step and the planarized layer lying at the top.
The CMP process is relatively unsuitable for both forms of further removal of material. Although the CMP process is highly topography-selective and is therefore eminently suitable for planarization steps, the process is frequently inefficient for the large-area, uniform removal of material from a surface which has already been planarized. It may even be disadvantageous in particular for material-specific removal, since at least the mechanical component of the CMP attacks all surface materials treated. In both cases, therefore, purely chemical etching steps are recommended, such as for example the technique known as etchback, wherein the surface which is to be machined is exposed to a suitable liquid composition of chemicals.
In mass production of electronic chips, in particular the CMP step is generally carried out batchwise, i.e. with simultaneous machining of a plurality of wafers. This leads to a very considerable time saving and therefore cost saving. Suitable multichamber and multihead installations are increasingly being used. Modern installations are designed in such a way that fluctuations in the rates at which material is removed between the different heads or chambers are very slight. However, these fluctuations, together with those of previous machining steps, such as for example trench etching or oxide deposition, may cumulatively amount to an order of magnitude which can no longer be reconciled with the evermore demanding tolerance requirements which result from the increasingly fine structures of the chips.
Therefore, in many cases installations wherein a measuring arrangement which is used to determine the fluctuations within a batch by measuring the layer thickness of each individual wafer is provided in the CMP area are in widespread use. The measurement results are used as a quality criterion to decide upon any remachining or, if appropriate, the particular use of the batch or individual wafers. However, as the tolerances are reduced, the scrap levels increase to an economically unacceptable degree.
A range of different configurations of the CMP processes used are known, a distinction substantially being drawn between four fundamental processes:                a. the conventional CMP process;        b. the fixed abrasive CMP process;        c. the electrochemical-mechanical deposition process;        d. the abrasive-free slurry process.        
In practice, the latter two processes are only relevant to the CMP processing of surfaces containing copper as conductive material and furthermore are still in the development stage. In contrast, the first two processes mentioned, i.e. the conventional CMP process and the fixed abrasive CMP process, are of general importance in particular for the processing of polysilicon oxide layers, tungsten and copper layers, wherein context the conventional CMP process is almost exclusively used, on account of the drawbacks of the fixed-abrasive CMP process.
When fabricating highly integrated circuits, the conventional chemical mechanical polishing (CMP) is in widespread use for the planarization of dielectrics or for the indirect patterning of wiring planes, i.e. for the removal of elevated regions of a patterned surface.
In the case of the conventional CMP process, a liquid which is mixed with polishing grains, preferably of a high hardness, and in some cases contains basic chemicals, known as the “slurry solution”, is introduced between that surface of a semiconductor wafer which is to be machined and a polishing pad.
The pad and the surface which is to be machined are in surface contact with one another and are moved relative to one another, so that the surface which is to be machined is abraded by the polishing grains moving between the two surfaces.
A topography selectivity is desired for efficient planarization of non-uniformly patterned surfaces. This means that more material should be removed from elevated regions than from regions lying at a lower level. In the case of chemical mechanical polishing, this cannot be ensured under all circumstances, in particular in the event of large and very small structures occurring together.
The polishing grains which move with the slurry solution can also penetrate into the lower-lying regions of the surface for material-removal purposes, so that overall complete planarization requires a greater amount of material to be removed than merely the layer thickness of the elevated structures.
In recent times, better results have been achieved by the process known as “fixed abrasive” CMP, wherein the polishing pad is covered with a polishing means, for example a polishing cloth, wherein the polishing grains are fixed in a polishing grain carrier and only project beyond the surface of the latter in certain regions. In the case of the fixed abrasive CMP, the polishing means and the surface which is to be machined are brought into contact with one another and are set in motion relative to one another. Depending on the specific device used, this can be effected by moving just one surface or both surfaces. In addition, if necessary it is possible to add suitable liquid chemicals in order to remove material by chemical means at the same time as by mechanical means. Since the polishing grains only interact with the surface that is to be machined at the actual points of contact between the polishing means and the surface which is to be machined, it is possible to achieve a particularly high level of topography selectivity by way of fixed abrasive CMP.
Strictly speaking, in a purely mechanical sense, the fixed abrasive CMP process is actually a grinding process rather than a polishing process, since the grinding or polishing grains cannot move freely, but rather are fixed in an unordered fashion in a carrier and in particular at the surface of the latter. Nevertheless, the term “polishing” has gained acceptance in general everyday usage and consequently it will continue to be used in this context.
It is inevitable that a number, in some cases a considerable number, of polishing grains will become detached from the carrier during the machining operation, depending on the type of wafer and/or polishing means, so that, on the one hand, a “true” polishing process also always takes place and, on the other hand, the polishing means becomes blunt or aggressive over the course of time, with the result that the amount of material removed per unit machining time drops or increases.
This effect is extremely undesirable in mass production, wherein a large number of wafers are successively subjected to the same CMP working step, since the same presettable parameters of a working step, such as for example machining time, chemicals selected, etc. would lead to different results depending on the degree of wear to the polishing means. Fluctuations of this nature cannot be tolerated in particular as the structures become ever smaller.
A phenomenon which has a similar result also occurs in the conventional CMP process explained above. However, the processes which lead to the blunting effect are different. In the conventional CMP process, the surface of the pad, which is actually elastic, “vitrifies”, i.e. the pores of the pad become blocked with relatively small polishing grains and in particular with material which has been removed from the surface that is to be machined. This leads to a hard and planar pad surface, with the result that significantly altered material-removal rates are produced. This discovery has generally been combated by cleaning and roughening the pad surface with the aid of a diamond needle. However, the process is too inaccurate for the fixed abrasive method and would cause the substantially pore-free polishing grain carrier to be destroyed, and consequently it is not suitable for use therein.
Therefore, that problem is currently attacked by exchanging the polishing means in steps, in each case before a new wafer is machined. Therefore, certain CMP devices offer automatic polishing-means advance (“roll-to-roll polisher”). However, equipment of this nature is expensive in two respects. Firstly, a device of this type requires considerable mechanical outlay. Secondly, it leads to excessive consumption of polishing means, giving rise to further costs. The polishing cloth which is customarily used has to satisfy extremely high accuracy demands with regard to its mechanical properties and with regard to the number, size and uniformity of the polishing grains, especially on account of the extremely small size of the structures which are to be machined. It is therefore complex and correspondingly expensive to produce.
However, the conventional CMP process has a range of drawbacks, such as for example what is known as the dishing effect, i.e. the undesirable recessing of surface structures, the relatively high consumption of slurry solution and the handling of the slurry solutions which are to be used. For example, the slurry solution has to be moved at regular intervals in order to achieve a uniform distribution of the suspended particles and to prevent the abrasive particles from settling. The abrasive particles used often have a mean diameter of 100 nm with a range from 40 to 200 nm, and consequently these particles have to be considered as macroscopic systems which are exposed in particular to the force of gravity and cannot be kept suspended by pure diffusion like microscopic particles.
Furthermore, in the case of existing slurry dispersions, the different interfacial properties of abrasive particles and slurry solution can lead to phase separation at excessively low temperatures, so that the slurry dispersion becomes unsuitable for use.
Certain drawbacks also have to be accepted in the fixed abrasive CMP process. For example, completely new equipment is required compared to the conventional CMP process. Furthermore, the fixed abrasive CMP process leads to high fluctuations in the material-removal rate, leading to an inhomogeneous surface treatment.